Wheels with spokes are known to serve for a long time. They essentially have three main components, namely, a hub, spokes and a rim. The hub, which is arranged in the center of the running wheel, is connected to the rim by a plurality of spokes. Tightening the spokes along the circumference of the rim with tensile force gives the wheel a high degree of rigidity. The spokes, which usually absorb only tensile forces, are hooked in the hub in eyelets or something similar, with typically a bend at the proximal (hub side) end. At its distal (rim-side) end they have a thread on the contrary. This works together with a so-called spoke nipple (or shortly nipple), which is rotatably mounted in the rim. The nipple has an internal thread, which can accommodate the distal external thread of the spoke. Turning the nipple changes the total length of the existing combination of nipple and spoke, which extends or shortens the spoke. That way, the spoke tension can change.
The change (initially in particular increasing) of spoke tension is not only initially necessary, e.g. for “lacing” of the first particulate wheel kit, but also for setting a desired spoke tension and in particular the so-called “centering”, which means the process of eliminating side impact and radial runout of the rim, as it occurs both after assembly of the running wheel, as well as through constant use. By centering, and thus tensing or relaxing the spokes, the tensile force on the rim is locally reduced or increased, which leads to a corresponding change in geometry.
Different tools are known for performing the centering operation.
When manually centering, the user controls visually, preferably with the aid of limit stops, sections of the running wheel for centering errors. A key or spoke wrench is used for a spoke tension, whereby the nipples of the respective spokes are rotated manually and the results are kept under a constant review. Therefore typically several wheel rotations are required until the result is satisfactory. Wheel alignment machines offer the possibility to fix a single wheel by its hub, and they have in the area corresponding to the rim limit stops, which allow for the convenient control of the lateral and radial runout. A disadvantage of such manual solutions, however, is the relatively high expenditure of time and the need for an appropriately skill user during the centering.
Therefore, partially or fully automatic tools have been developed to carry out the control of lateral and radial runout and the resulting adjustments automatically. Such a device is known for example from the document WO 2006 114087 A2. This includes an axle support for a running wheel, a drive roll for the same, a device for detecting local lateral and radial runout, as well as a device for the automated spoke nipple rotation. The measured values of the rim are transmitted to a processing unit, which determines the proper rotation direction and number of rotations. These values are then used to control the automatic turning of the spoke nipple.
A fundamental problem occurring in changing of the spoke tension is the presence of frictional forces on the thread. To rotate the nipple relative to the spoke, at first, the static friction of the thread has to be overcome. Both here and in the subsequent sliding friction, the attrition forces can counteract a movement of the nipple in the thread, so that not only the nipple but also the spoke itself rotates around its longitudinal axis, whereby a torsional stress is generated. The spoke twists until its “breakaway” and can now accommodate pure tensile forces due to the torque acting on the spoke nipples. A twisted spoke is undesirable for several reasons: for example, the torsion leads to a sub-optimal resting of the proximal end in the eyelet of the hub, which can lead to a spoke breakage. As another example, a twisted spoke tends, during strong vibrations that typically occur during driving, again in the repeated spontaneous “breakaway” from the nipple, resulting in an unscrewing and simultaneous relaxation of the spoke; an unintended reduction of the spoke tension and an increase in the lateral or radial runout are the result. Even in case when the twisted spoke along with the nipple performs a common, torsion reducing rotary motion (e.g. due to vibration), there is an extension of the (then a little less twisted) spoke as a result.
In order to avoid this effect, or at least to minimize it, experts deliberately slightly widen (“overturn”) the rotational movement of the nipple than initially necessary, and immediately afterwards execute a slight rotational movement in the opposite direction.
During the minor reverse rotation the spoke indeed does not change its position in the thread of the nipple due to the thread friction, but it is brought from the first twisted location in a “neutral” position in which only tensile forces act. It is clear that this method requires an appropriate experience of a manually working user. The above-mentioned automated device is also able to perform appropriate compensating movements, but it also increases time and computational effort, especially since in a non-twisted spoke, the spoke tension can be determined in a particularly simple manner. A most uniform tension of all spokes in turn is a sign of quality with regard to a sustainable balance of forces of the running wheel with a lower tendency to axial and radial offsets as well as increased stability of the spokes.
Another way to reduce the adverse effect of the frictional forces in the threads and the related co-rotation of the spoke about its longitudinal axis is disclosed in document WO 2008 007 954 A1. This publication describes an automatically working centering device for running wheels with spokes. This includes a clamp for a spoke so that the spoke cannot rotate during the rotation of the spoke nipple. This already leads to an improvement of the working result.